The present invention pertains generally to the field of stent securing catheters, and, in particular, to a stent securing catheter having a tapered distal end.
Stents are increasingly used in Percutaneous Transluminal Coronary Angioplasty (PTCA). PTCA is a well established procedure for dilating stenosed vessel regions in the heart. In this procedure, a balloon angioplasty catheter is introduced into the vasculature, typically through an incision in the femoral artery in the groin. The balloon catheter is advanced through the femoral artery, through the aortic arch, and into the artery to be treated. The balloon is advanced across a lesion and inflated, dilating the vessel at the location of the balloon expansion. The dilation increases the vessel cross sectional area and the resultant blood flow.
Over a period of time, the dilated vessel section may narrow again, in part due to a rebound from the angioplasty procedure, thereby reversing some of the benefits of the angioplasty. To prevent this vessel narrowing, stents are increasingly used. Stents are placed across the dilated region and radially expanded, opposing any inward radial force by the vessel walls.
Stents may be categorized as self-expanding and balloon expanding. The self-expanding stents are contained within a sheath to prevent premature expansion. The stent is placed within a guide catheter, moved across a lesion, withdrawn from the sheath, and the stent, being biased to expand, expands, ideally with sufficient force to resist the vessel wall rebound force which can occur after angioplasty. The stent can be left in place indefinitely.
Balloon-expandable stent deployment requires "tacking up" the stent, forcing stent struts or members radially outward into close proximity with the vessel wall. The stent is mounted over an uninflated balloon, crimped, and the balloon with stent advanced within a guide catheter. The balloon with stent is advanced distally out of the guide catheter across the lesion. The balloon is inflated, expanding the stent, thereby tacking the stent in place. For optimal stent placement, it is necessary for the stent to be properly positioned axially on the balloon prior to balloon inflation. A non-compliant balloon operated at high pressure is typically used to expand the inside diameter of the stent, forcing it against the vessel interior walls. The ballcon is deflated, and withdrawn proximally into the guide catheter.
For both types of stents, self-expanding and balloon expandable, a sheath can be used during stent delivery, being interposed between stent and guide catheter. The sheath adds a not-insubstantial thickness around the stent increasing the vessel inside diameter required to pass the sheathed stent. In one case, when using a sheath, a sheath outside diameter of 72 mils (thousands of an inch) is required to place a stent having an outside diameter of 60 mils. When not using a sheath, clearance is only required for the 60 mil sheath. This reduced outer diameter translates into increased vasculature accessible for stent placement, making treatable otherwise untreatable lesions.
As a result of the increased vasculature reachable without sheaths, treating physicians increasingly prefer to place stents without using a sheath, the "bare mounted" technique. This is possible with balloon-expanding stents, but has associated difficulties. A stent can be bare mounted over a balloon, crimped, and the balloon advanced through the guide catheter to the distal region of the guide catheter, which is positioned proximal to the vessel region having a lesion. The balloon with stent is advanced distally out of the guide catheter and across the lesion. When the stent is crimped onto the balloon, there can be a slight recoil, such that when balloon and stent are advanced out of the guide catheter, the stent is too large to be retracted into the guide catheter even before balloon inflation.
Occasionally, there are situations where the stent becomes partially or totally dislodged from the balloon. The dislodged stent may be detected while still within the guide catheter. A dislodged stent can be detected using radiography, observing relative positions of radiopaque regions on the stent and balloon catheter. When the stent is dislodged while within the guide catheter, it may be possible to withdraw the balloon catheter and stent together.
At other times, the stent becomes dislodged after the stent has been advanced out of the guide catheter. As the balloon-expanding stents do not self-expand, this creates the situation where a stent may become loose in the vasculature. When the stent is only partially dislodged from the balloon, the balloon with partially mounted stent may be withdrawn proximally into the guide catheter. The stent outer diameter is often only slightly less than the inner diameter of the guide catheter, to keep the guide catheter size down and increase the amount of vasculature open to the guide catheter. The stent outer diameter may be larger than the guide catheter inner diameter, and withdrawal of the balloon will not withdraw the stent, but may instead force the stent off the balloon.
In cases where the stent remains sufficiently small to fit within the guide catheter, withdrawal may still prove problematic. During attempted recovery, there is a point at which the proximal edge of the stent is to be withdrawn proximally past the distal edge of the guide catheter. If the stent is not centered relative to the longitudinal axis of the guide catheter, the guide catheter distal edge may catch against the stent proximal edge, forcing the stent from the balloon.